U.S. patent application number 15/936718 was filed with the patent office on 2019-10-03 for cutting tool having siliconized silicon carbide shank connected to diamond cutting head via vacuum-brazed thermal interface.
The applicant listed for this patent is WolvertonBailey Tech Fund 1, LLC. Invention is credited to James A. Austin.
Application Number | 20190299297 15/936718 |
Document ID | / |
Family ID | 68055269 |
Filed Date | 2019-10-03 |
![](/patent/app/20190299297/US20190299297A1-20191003-D00000.png)
![](/patent/app/20190299297/US20190299297A1-20191003-D00001.png)
![](/patent/app/20190299297/US20190299297A1-20191003-D00002.png)
![](/patent/app/20190299297/US20190299297A1-20191003-D00003.png)
United States Patent
Application |
20190299297 |
Kind Code |
A1 |
Austin; James A. |
October 3, 2019 |
CUTTING TOOL HAVING SILICONIZED SILICON CARBIDE SHANK CONNECTED TO
DIAMOND CUTTING HEAD VIA VACUUM-BRAZED THERMAL INTERFACE
Abstract
A rotating cutting tool having a shank composed of siliconized
silicon carbide (SiSiC), and a diamond cutting head that is
mechanically connected to and in thermal communication with the
shank via a vacuum-brazed thermal interface.
Inventors: |
Austin; James A.; (Lompoc,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WolvertonBailey Tech Fund 1, LLC |
Santa Rosa |
CA |
US |
|
|
Family ID: |
68055269 |
Appl. No.: |
15/936718 |
Filed: |
March 27, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23B 2226/72 20130101;
B23B 27/20 20130101; B23B 2240/08 20130101; B23B 2226/31 20130101;
B23B 27/10 20130101; B23B 27/148 20130101; B23B 2250/125
20130101 |
International
Class: |
B23B 27/14 20060101
B23B027/14 |
Claims
1. A cutting tool for ultra-precision machining, comprising: a
cutting tool head having a cutting edge to contact a workpiece
during operation of the cutting tool, the cutting tool head being
composed of a single-crystal diamond material; a shank having a
shank body composed of siliconized silicon carbide; and a thermal
interface composed of a metal material to form a vacuum-brazed
connection between the cutting tool head and the shank body, the
thermal interface being configured for thermal and physical contact
between the cutting tool head to the shank body, wherein the
thermal interface is configured to serve as a thermal conductor to
transfer heat from the cutting tool head to the shank body, and the
shank body is configured to act as a heat sink to transfer heat
from the thermal interface in order that an operational temperature
of the cutting edge is less than 482.degree. C.
2. The cutting tool cutting tool of claim 1, wherein the cutting
tool head is mechanically connected to and in thermal communication
with the shank via a vacuum-brazed thermal interface.
3. The cutting tool cutting tool of claim 1, wherein the cutting
tool head contacts the holder body at a single plane of contact
interface where atomic bonds are shared.
4. The cutting tool cutting tool of claim 1, wherein the metal
material has a thermal conductivity that is greater than the
thermal conductivity of a holder material but less than the thermal
conductivity of diamond.
5. The cutting tool cutting tool of claim 1, wherein the metal
material comprises a silver alloy.
6. A cutting tool for ultra-precision machining, comprising: a
cutting tool head having a cutting edge to contact a workpiece, the
cutting tool head being composed of diamond; a cutting tool head
holder upon which the cutting tool head is mounted, the cutting
tool head holder being composed of a holder material having a
thermal conductivity of at least 225 W/m.sup.2K and a modulus of
elasticity of not less than 340 GPa; and a thermal interface
configured for thermal and physical contact between the cutting
tool head to the cutting tool head holder, the thermal interface
being composed of a thermal conducting material to form a
vacuum-brazed connection between the cutting tool head and the
cutting tool head holder, and to also transfer heat from the
cutting tool head to the cutting tool head holder in order to
maintain an operational temperature of the cutting edge at less
than a predetermined temperature.
7. The cutting tool of claim 6, wherein the holder material
comprises siliconized silicon carbide.
8. The cutting tool of claim 6, wherein the predetermined
temperature comprises 482.degree. C.
9. The cutting tool of claim 6, wherein the cutting tool head
contacts the cutting tool head holder at a single contact
interface.
10. The cutting tool of claim 6, wherein the thermal conducting
material comprises a metal material.
11. The cutting tool of claim 10, wherein the metal material has a
thermal conductivity that is greater than the thermal conductivity
of the cutting tool head holder and less than the thermal
conductivity of diamond.
12. The cutting tool of claim 11, wherein the metal material
comprises a silver alloy.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 62/477,224 (filed Mar. 27, 2017), which is hereby
incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] Embodiments relate to a cutting tool having a shank composed
of siliconized silicon carbide (SiSiC) that is connected to diamond
cutting head via a vacuum-brazed thermal interface.
BACKGROUND
[0003] When single-crystal diamonds are used to manufacture
components specifically within the ultra-precision (e.g., to
produce mirror-like surface quality and having exacting surface
accuracy) machining disciplines referred to as diamond-turning and
diamond-flycutting, the acceptable wear mechanism at the cutting
edge is "graphitization," which may only occur in the presence of
oxygen and at temperatures above 482.degree. C. (900.degree.
F.).
[0004] From the publication "The Kinetics of the Diamond--Oxygen
Reaction" by T. Evans and C. Phaal, from the Physics Department,
University of Reading, England (Aug. 8, 1961), one may understand
that: (1) single-crystal diamonds wear via "graphitization" in an
experiment where diamonds of {100} and {110} crystallographic
orientation were subjected to a hot (700.degree. C.) stream of
purified oxygen gas, directed parallel across the crystal faces,
and (2) the {100} plane is more resistive to this parallel,
"sliding" wear mechanism than is the {110} plane.
[0005] In one instance, the graphitization temperature threshold
was discovered to be 482.degree. C. (900.degree. F.), and that for
the perpendicular (oxygen streaming directly at the test crystal)
"impinging" wear mechanism, the {110} plane was more resistive to
wear than the {100} plane.
[0006] In the publication "F2, H2O, and O2 etching rates of diamond
and the effects of F2, HF and H2O on the molecular O2 etching of
{110} diamond" by C. J. Chu et al. of Rice University (Tex.)
Department of Chemistry, discloses that graphitization initiates at
some temperature of less than 600.degree. C.
[0007] Conventional machining takes place in a machine shop, and is
generally performed on lathes, mills and other equipment at room
temperature and normal atmospheric pressure. A machinist will
set-up, program, initiate, and monitor the machining process, and
are required to remain in the vicinity of their equipment during
the machining process.
[0008] When a machinist needs a cutting tool, an appropriate
tooling bar or shank will be required in which to an appropriate
cutting head, tip, or insert (having a cutting edge that is to dig
into the workpiece) is affixed to the shank/tooling bar. The shank
with the affixed cutting insert is then fastened to a tool post of
the lathe. This is how a cutting edge is attached to a conventional
lathe that directs the cutting tips' cut depth and toolpath.
[0009] In traditional machining best practices, the shank/tooling
bar is used for positioning, anti-chatter, and anti-vibration
during operation of the cutting tool. The shank/tooling bar must be
rigid, i.e., have a modulus of elasticity that is large in
magnitude so that cutting edge vibration or "chatter" is eliminated
or at least minimized. A shank/tooling bar composed of tungsten
carbide (WC) is conventionally used for machining operations
requiring optimized stiffness, and is also inexpensive. The
technology and science of tungsten carbide is so highly advanced
that the insert can also be made of solid "carbide." In this
regard, it bears noting that a variant of tungsten carbide is
available (deemed HD 17.7 Tungsten) that is "enhanced" to provide
thermal conductivity as high as 113 W/m.sup.2K.
[0010] The insert is attached mechanically by a single screw, which
is acceptable for traditional machining, but is not optimal for
precision diamond machining. Single screw mounting cannot guarantee
that there will be a solid, non-yielding, non-vibrating attachment
of the insert to the shank.
[0011] In conventional machining, keeping the cutting insert and/or
the workpiece from overheating during the machining process is
mitigated via application of large volumes of high-pressure,
refrigerated coolant utilized to extend the life of the cutting
edge of the diamond insert. A science has developed to encompass
all the technical discoveries with respect to the chemistry and
temperature of the coolants/lubricants and the methods of their
application/delivery during the conventional machining process. In
diamond-turning and flycutting, coolants (including just directing
a low-pressure stream of air across the cutting edge) and
lubricants applied as a mist, must be applied carefully. Since a
typical depth of cut for diamond-turning and flycutting of 2
microns (80 millionths of an inch) is very shallow, the use of
cold, fluid coolant at the cutting edge to the workpiece interface
(as in traditional machining) is undesirable in that it may result
in vibration. Such vibration may in turn cause higher values of
roughness, and/or thermal shrinkage of the workpiece. Consequently,
this will result in unwanted figure accuracy variations. Not only
is figure accuracy in jeopardy, but surface quality is also
diminished. The dynamics of material removal, especially when
diamonod-machining engineering crystals, begins to degrade as the
cutting edge loses its sharpness. This normally requires the
operator to intervene and change to a fresh, sharp diamond cutting
tool.
[0012] In diamond-turning and flycutting operations, surface finish
specifications are measured in Angstroms. One reason that a solid,
single-crystal diamond (about the size of a one-third carat
engagement ring stone) is used as the cutting head is that the
surface quality (roughness) requirements are very demanding in
diamond-turning and flycutting. An "8" (8 microinch Ra or roughness
average) finish is very smooth for traditional machining, whereas
20 Angstroms Rq (less forgiving than Ra), (or what would be a
"0.08" machinist finish, two orders of magnitude better) is good
surface quality for diamond turning. As a result, it must be
understood that the "diamond" in diamond-turning is used because it
can be sharpened/polished to the degree that a typical callout for
good sharpening quality is "chip free edge at 800 power
magnification." In fact, in some instances, a 90 degree section of
cutting edge can be chip-free at 20,000 power magnification (as
measured by an electron microscope).
[0013] A diamond head with any measureable wear is quickly
approaching the point where it must be removed in favor of a sharp
one. The erosion at the top edge and front of the diamond cutter,
as the result of graphitization, causes the graphitized surfaces to
physically recede, and thus, effect the surface figure accuracy of
the workpiece (the diamond cutting edge is no longer in its
programmed position). When the worn area of the cutting edge (from
the front) is inspected in electron microscopes or other
high-powered microscopes using differential interference contrast,
it appears as a microscopic veil or a waterfall.
[0014] The front (or clearance face) of the diamond is typically
either a cylinder or a cone, and the top (or rake face) of the
diamond is usually a flat plane, so the intersection of the curved
front and the flat top of the diamond is a segment of a circle or
an ellipse. The clearance face and the rake face can be ground and
polished by skillful technicians so that the cutting edge is within
+/-1 microinch of a perfect circle.
[0015] The diamond head or insert must be removed and replaced with
a new or re-sharpened one should it suffers a chip along the
cutting edge. The diamond cutting edge is useless if it suffers
even one small chip that is one micron in size. This is because a
small chip in the cutting edge would cause a repetitive peak and
valley in the surface of the workpiece being diamond-machined,
which would immediately violate the surface finish specification of
the customer.
[0016] There are numerous small chips (greater than one micron in
size) in the cutting edge of a conventional machinists cutting
head, as it comes from the vendor as brand-new. Neither is the
accuracy of any radius as near perfect.
[0017] Diamond-turning surface figure requirements are measured in
fractions of a wavelength of red laser light (one wavelength of red
laser light.apprxeq.25 millionths of one inch). A typical blueprint
callout for diamond-turned optical surface figure accuracy is 1/4
of that 25 millionths (i.e., "quarter wave"), but some customers
require 1/10 wave. "Wave" is parlance for one wavelength in the
industry. So, a manufacturer of diamond-turned components must
understand that its error budget is almost always less than 25
millionths of an inch.
[0018] A non-traditional machining process, diamond machining
(including ruling, turning and fly cutting) where a sculpted
single-crystal diamond is the cutting edge, originated in
traditional machining, but quickly outgrew it. The use of
traditional ball or roller bearing spindles were eliminated in
favor of air-bearing spindles with 1 microinch total run-out
(roundness). Oil-hydrostatic slideways replaced traditional bearing
slides that require some "clearance or looseness" in order to allow
for movement. Allowing even 1 micron of looseness, in roller or
ball bearing applications in a diamond-turning machine, would
defeat the very tight tolerances that customers require. For the
same reason, leadscrew slide-drive mechanisms were replaced with
non-contact, linear motors (magnetic drive devices). The
air-bearing spindles are even driven by integrally mounting the
permanent magnets of a brushless DC motor directly to the rotating
element of the spindle, so that the driving force causing rotation,
is purely magnetic.
[0019] Diamond machining is also non-traditional because it
requires that the process, and even the machine's temperature, be
held at .+-. (plus or minus) 1.degree. F. or better. This is
important because the required tolerances for figure accuracy and
surface quality of the components produced by diamond-turning are
much more stringent. Tight figure accuracy tolerances in
traditional machining may be +/- two ten-thousandths of an inch or
0.0002 inches, but (as described herein) in diamond turning, +/-6
microinches (0.000006'') is a more normal tolerance, with +/-2
microinches (millionths of an inch) as a tighter tolerance
sometimes required by the customer. Steel (of which the
diamond-turning machine is mostly made) expands 7 millionths of an
inch per .degree. F. per inch of thickness. Aluminum (a popular raw
material for telescope mirrors) expands 13 millionths of an inch
per .degree. F. per inch of thickness. This is why the
diamond-turning process must be tightly controlled from a thermal
perspective. If the workpiece being machined is allowed to shrink
and swell to a certain degree due to thermal expansion and
contraction of anything in the process (including the
diamond-turning lathe itself), then those tight tolerances may be
violated.
[0020] Another fact that sets diamond-machining apart is that
acceptable diamond head edge wear is only as a result of
"graphitization." This only occurs when the outermost (surface)
molecules of a diamond are exposed to moving material contact
(friction), causing localized temperatures exceeding 900.degree. F.
(482.degree. C.) in the presence of oxygen. The word "localized" is
important to understand. Even though 482.degree. C. is being
developed by the cutting process, the area of contact is so small
and localized that the heat generated there does not contribute to
thermal expansion of the workpiece. The heat generated at the tool
head-to-part interface is more quickly spread to the volume of the
diamond (which is the best conductor of heat of all known materials
at 2200 W/m.sup.2K).
[0021] The temperature of the diamond head where the workpiece and
the chips contact the diamond head may exceed 482.degree. C.
Diamond converts to graphite above 482.degree. C., and in the
presence of oxygen (O.sub.2), the graphite is swept away by the
process (graphite does not convert back to diamond). Just one pass,
removing 2 microns from one side of a 2'' diameter, 1/4'' thick
piece of copper, is enough to develop microscopic but measurable
wear on the rake face, cutting edge, and clearance face of the
diamond head. So the diamond head exhibits graphitization at these
areas where the operational temperature exceeds 482.degree. C.
Since a diamond head with a shank composed of HD 17.7 Tungsten
material (having a thermal conductivity of 113 W/m.sup.2K) begins
to graphitize and develop wear areas via cutting friction/heat,
heat is not transferred from the diamond head quickly enough from
the contact area, and thus, builds up in the diamond head. This
heat build up results in graphitization, which makes the diamond
head cutting head lose its sharpness. Please note that a "sharp"
diamond head cutting edge has a 2 microinch (2 millionths of an
inch) radius at its cutting edge. Only single-crystal diamond can
be made this sharp and chip-free.
[0022] The thermal conductivity of (non-enhanced) Silicon Carbide
is 100 W/m.sup.2K. It has industrial applicability as a heat sink,
but has not been tested as a diamond head shank. It also does not
work to limit graphitization of diamonds. For instance, even
single-crystal SiC is not a sufficient heat sink to limit diamond
head wear. Testing has revealed another enhanced SiC material that
has a thermal conductivity of 150 W/m.sup.2K.
[0023] A simplified explanation of graphitization is provided here.
As the cutting edge of the diamond head contacts/enters the
workpiece, in order to remove some material, friction and chip
deflection friction must necessarily occur. Accordingly, wear of
the diamond head occurs at the cutting edge, but also on the rake
face surface behind the cutting edge, where the material being
removed contacts the top of the diamond head and is deflected away.
During operation, the diamond head may travel at speeds of 78 ft/s,
such that frictional heat is generated during at the contact
area/region (141.5 microns or 0.006'' in length) between the
diamond head and the workpiece. Contact, and therefore, friction
occurs behind and below the cutting edge, and significant heat is
thereby generated.
[0024] It is important to note that the amount of operational wear
(e.g., just a few tenths of a micron) experienced by the diamond
head cutting edge means the once-sharp cutting edge is now becoming
dull. In turn, replacement of diamond head is required since a dull
cutting edge will produce higher values of roughness, and the
diamond will wear faster as the edge dulls. As the cutting edge
wears, more contact surface area of the cutting head and the
workpiece results. More surface contact means more heat will be
generated due to friction, yielding more wear.
[0025] Scientists, engineers, and machinists have worked on the
problem of limiting graphitization in different ways. One attempted
solution involved immersing the workpiece, the diamond head, the
shank, and a portion of the post in liquid nitrogen (LN2). While
this served to significantly reduce graphitization, it is
impractical. Moreover, although graphitization can be controlled
where immersion in LN2 keeps the temperature down and the oxygen
away, the reduction in area of everything immersed in LN2 (due to
shrinkage) due to the super-cold LN2, results in geometric
accuracies being uncontrollable.
[0026] Other solutions involved the application of refrigerated
coolants and carbon dioxide "snow", but were unsuccessful. With the
application of other cold coolants via spray nozzle (out into a
machine in a normal room environment), heat suppression was
insufficient and there was always the presence of oxygen, allowing
for graphitization of the diamond. And even in this latter case,
figure accuracy of the workpiece still suffered due to drastic
thermal variances.
[0027] Due to customer demand and requirements, something is needed
to minimize or otherwise eliminate (e.g., to less than +/-1
microinch) operational wear of the cutting edge of the diamond.
SUMMARY
[0028] Embodiments relate to a diamond cutting head having a shank
and/or insert base composed of a material that is to minimize or
otherwise eliminate operational diamond head edge wear. In
accordance with embodiments, such a material comprises siliconized
silicon carbide. The importance of accuracy in the
diamond-machining process results in hours being extended to
properly set-up a diamond head, making it ready for production of
aspheric optics. Limiting diamond head edge wear would reduce or
otherwise eliminate the need to replace the diamond cutter so
frequently.
[0029] Embodiments also relate to a diamond cutting head having a
shank and/or insert base composed of a material that is to make
practical the production of larger scale silicon aspheric
optics.
[0030] Embodiments also relate to a diamond cutting head having a
shank and/or insert base composed of a material that is beneficial
for use by traditional machinists.
[0031] During operation of the cutting tool, frictional heat
generated by contact between the cutting tool edge and the
workpiece is drawn away from the diamond cutter and distributed to
the shank via a thermal interface. The diamond cutter has a very
small area thereon where contact is made with the workpiece being
diamond-turned. At this contact area, heat is generated and spreads
quickly to the bulk of the diamond due to the thermal conductivity
of diamond being 2200 W/m.sup.2K. For the diamond to braze
junction, one heat transfer equation for diamond shows: Q dot (heat
flow)=K.times.A.times..DELTA.T, where K is the heat transfer
coefficient of diamond (2200 W/m.sup.2K), A (which A represents the
braze contact area of the diamond to the shank) is about 5 square
millimeters (5 millionths of a square meter), the temperature of
the diamond head being 482.degree. C., and the temperature of the
diamond-to-shank junction area at 200.degree. C. Using this data
results in 3.1 watts being transferred at the junction. There is a
one micron thick layer of vacuum braze interface, e.g., silver
solder, which is configured to atomically bond the diamond cutter
to the shank and serves to transfer heat from the diamond cutter to
the shank. The heat transfer coefficient of the vacuum braze
interface is 400 W/m.sup.2K. Because embodiments provide for a thin
layer of the vacuum braze interface, it may not need to be
considered in the heat transfer equation.
DRAWINGS
[0032] Embodiments will be illustrated by way of example in the
drawings and explained in the description hereinbelow.
[0033] FIG. 1 is a perspective view of an embodiment of a cutting
tool.
[0034] FIG. 2 is a side sectional view of the cutting tool, in
accordance with embodiments.
[0035] FIG. 3 is a block diagram of the cutting tool, in accordance
with embodiments.
DESCRIPTION
[0036] Important terms:
[0037] Graphitization: The result of heating the exposed surface of
a diamond, in the presence of oxygen, to 900.degree. F.
(482.degree. C.) via rubbing or friction contact. This specific,
hot, impinging, and/or sliding action causes the exposed diamond
molecules, in the area of contact, to physically convert from
diamond to the stable form of carbon, which is graphite. Once
converted to graphite, it is swept away by the friction action,
exposing fresh diamond molecules below it. Graphite does not
re-convert to diamond.
[0038] Coefficient of Thermal Conductivity or Heat Transfer
Coefficient: measured in Watts per square Meter Kelvin
(W/m.sup.2K), and is a measure of how fast heat flows through a
given material, from where heat is generated to where heat is
extracted. Larger values equates to faster heat flow.
[0039] Siliconized Silicon Carbide (SiSiC): a solid material based
on silicon carbide (SiC), and is enhanced with 15% pure silicon to
produce a significantly greater thermal conductivity (225
W/m.sup.2K) than just pure silicon carbide.
[0040] Modulus of elasticity: the measure of a materials'
elasticity, lower numbers indicate very elastic (bendable), higher
numbers indicate a lack of elasticity (rigidity or stiffness),
measured in Giga-Pascals (GPa).
[0041] Diamond: the solid, single piece having a cutting edge
skillfully sculpted, and includes a flat surface specifically
produced to provide a mating interface to the shank. The diamond
must be skillfully selected for exceptional quality and
crystalographically oriented for optimum cutting edge performance.
Internal flaws and one specific orientation {111} must be
avoided.
[0042] Tungsten Carbide (non-enhanced): a man-made material
(thermal conductivity of 83 W/m.sup.2K) used extensively where high
rigidity is desired (Modulus of elasticity=500 GPa). The industrial
nickname for this material is "Carbide."
[0043] Silicon Carbide (non-enhanced): SiC is a man-made powdered
material that when formed into a solid is used as a heat sink
(thermal conductivity of 100 W/m.sup.2K).
[0044] Thermal expansion and contraction: measured in microinches
per .degree. F., per inch of material thickness. Most materials
expand when warmed and contract when cooled. Each material has a
known amount of expansion/contraction. This is stated as the
materials "coefficient of thermal expansion" or CTE. For example,
Steel shrinks or expands 7 millionths of an inch per .degree. F.
per inch of thickness. Aluminum's CTE is 13 millionths of an inch
per .degree. F. per inch of thickness.
[0045] Surface Figure Accuracy: The deviation from the perfect,
exact, mathematical model, and the actual surface profile produced.
Measured in fractions of a wavelength of red laser light.
[0046] Surface Roughness (Rq): The microscopic RMS surface texture,
with its peaks and valleys, averaged over an area of about 3/4 of a
square millimeter, measured in Angstroms because that is how smooth
the customer needs it to be.
[0047] Special units of measure:
[0048] 1 "Microinch" equals 1 millionth of 1 inch.
[0049] 1 "Micron" (one one-thousandths of a millimeter)
approximately equals 40 millionths of 1 inch.
[0050] 1 "Angstrom"=1.times.10.sup.-10 meter (or one ten billionth
of a meter). 254 Angstroms equal 1 microinch.
[0051] 1 wavelength or "Wave" of red laser light approximately
equals 25 microinches.
[0052] Temperatures measured in either Kelvin or the Celsius scale
are interchangeable in mathematical calculations.
[0053] As illustrated in FIGS. 1 to 3, embodiments relate to a
cutting tool 10 to perform ultra-precision machining of a
workpiece. The cutting tool 10 includes a cutting tool head 11
having a cutting edge 11a to contact the workpiece during a
machining operation. In accordance with embodiments, the cutting
tool head 11 comprises a single-crystal diamond material.
Embodiments are intended to cover diamond-machining processes
performed at or near room temperature and atmospheric pressure, and
in the presence of oxygen.
[0054] A shank 12 is specifically configured to serve as a heat
sink which transfers heat H generated by frictional contact between
the cutting edge 11a and the workpiece. The shank 12 comprises a
shank body connected at one end to a tool post 13 and another end
to the cutting tool head 11. In accordance with embodiments, the
shank is composed of a material having a predetermined thermal
conductivity that permits the transfer of heat from the cutting
tool head 11 at a rate sufficient to prevent the operational
temperature of the cutting tool edge 11a to exceed 482.degree. C.
This advantageously prevents undesirable graphitization, reduces or
otherwise eliminates operational wear of the cutting edge 11a
caused by graphitization, thereby extending the operating life of
the cutting tool head 11. In accordance with embodiments, the
predetermined thermal conductivity of such a material is 225
W/m.sup.2K. The material may, for example, comprise siliconized
silicon carbide (SiSiC). The chemical composition of the shank 12
may be, for example, approximately 85% pure silicon carbide, and
approximately 15% pure silicon (with less than approximately 0.05%
other materials). The modulus of elasticity the shank 12 should not
be less than 340 GPa and its thermal conductivity should not be
less than 225 W/m.sup.2K. This is in opposition to use of an
enhanced tungsten shank that has a maximum thermal conductivity of
113 W/m.sup.2K.
[0055] Taking into account a 200.degree. C. input, the shank 12,
which is composed of SiSiC, is configured to distribute the
frictional heat to the tool post, through 1.125 square inches of
contact surface, thereby heating the junction surfaces to
30.degree. C. The SiSiC shank dissipates approximately 27.76 W of
heat. 225.times.1.125.times.170/1550 (1550 square inches to a
square meter)=27.76 W dissipated.
[0056] Accordingly, use of a shank material having a thermal
conductivity that prevents the cutting edge 11a from reaching a
predetermined threshold temperature of 482.degree. C. 225
W/m.sup.2K will serve to prolong the operational life of a diamond
head used for a precision cutting tool. There is a "breakpoint"
somewhere between 113 W/m.sup.2K and 150 W/m.sup.2K, where heat
finally travels fast enough so that the diamond head is not heated
to the point where the diamonds' cutting edge 11a attains a
temperature of 482.degree. C.
[0057] As illustrated in FIGS. 2 and 3, a thermal interface 14 is
arranged between the cutting tool head 11 and the shank 12. The
thermal interface 14 is configured to form a thermal and physical
connection between the cutting tool head 11 and the shank 12. In
accordance with embodiments, the thermal interface 14 is configured
to serve as a thermal conductor which transfers the frictional heat
from the cutting tool head 11 to the shank 12 via heat
conduction.
[0058] In accordance with embodiments, the thermal interface 14 is
composed of a thermal conducting material, such as, for example, a
metal material. Such a metal material may comprise, for example, a
silver-based alloy which is vacuumed brazed to form a vacuum-brazed
connection between the cutting tool head 11 and the shank 12. The
metal material should have a thermal conductivity that is greater
than the thermal conductivity of the shank 12 but less that the
thermal conductivity of the diamond cutting tool head 11. In that
way, the thermal interface 14 is configured to quickly transfer
heat H from the cutting tool head 11 so that an operational
temperature of the cutting edge 11a does not reach or go above the
threshold temperature of 482.degree. C. when contacting the
workpiece. In accordance with embodiments, a heat-conductive paste
is to be applied at any boundary(ies) between the shank 12 and the
lathe where heat transfer can be enhanced. The cutting tool head 11
is configured to contact the shank 12 at a single plane of contact
interface where atomic bonds are shared.
[0059] In operation, the cutting tool 10 is to perform such that
heat H is transferred from the diamond cutting tool head 11 to the
thermal interface 14, then to the shank 12, and then to the
tool-holder/tool post 13.
[0060] Practice of embodiments provide for numerous technical
advantages. For instance, use of an enhanced SiSiC shank 12 results
in quick removal of heat H generated by the diamond cutting tool
edge 11a during operation of the cutting tool 10. In that way, the
temperature of the cutting edge 11a does not reach or exceed the
threshold temperature of 482.degree. C. when contacting the
workpiece. Consequently, operational wear caused by graphitization
is significantly limited or otherwise eliminated. This thereby
extends the operating life of the cutting tool 11, and reduces
overall maintenance costs connected to the replacement of cutting
tools due to the fact that the entire tool 10 must be removed and
sent to the sharpening service. The head is not removed from the
shank to facilitate sharpening.
[0061] The shank 12, due to it comprising a SiSiC material, limits
graphitization of all single-crystal diamonds, natural mined
stones, or synthetically grown diamonds. Polycrystalline (PCD) and
Chemical Vapor Deposition (CVD) diamond tooling will also be
benefitted. It bears noting that PCD diamond heads (and so the
cutting edge) cannot be sharpened to much better than "chip free at
250 power magnification" due to 1/3 of the diamond powder being
oriented in the {111} cleave plane.
[0062] The cutting tool 10 advantageously has a design in which the
diamond head 11 and the shank 12 are connected at a single contact
surface by a vacuum-brazed connection that also serves as a thermal
interface 14. This serves to quickly transfer heat H from the
cutting edge 11a of the diamond cutting tool head 11 to the heat
sink (i.e., shank 12). The shank 12 may then transfer the heat H to
the tool-holder/tool post 13.
[0063] In accordance with embodiments, the diamond cutting tool
head 11 must be vacuum-brazed to the shank 12, and not sintered.
Sintering is a mechanical capture method which does not permit the
sharing of atomic bonds, and thus, allows the diamond cutting tool
head 11 to become loose in its mount at a microscopic level,
significantly impairing its ability to produce minimized roughness
and accurate surface figure. In accordance with embodiments,
therefore, the diamond cutting tool head 11 is configured to
contact the shank 12 only through the braze media of the thermal
interface 14 therebetween, where atomic bonds are shared between
the diamond cutting tool head 11 and the shank 12. The thermal
conductivity of the silver-braze media is greater than that of the
shank 12, but less than that of the diamond cutting tool head
11.
[0064] In accordance with embodiments, diamond-machining processes
where air, coolants, and/or lubricants are directed into the
machining interface may be used in conjunction with the cutting
tool 10 to further enhance the transfer of heat (about 20
Watts/m.sup.2K).
ADDITIONAL NOTES AND EXAMPLES
[0065] Example One may include a cutting tool for ultra-precision
machining, comprising: a cutting tool head having a cutting edge to
contact a workpiece during operation of the cutting tool, the
cutting tool head being composed of a single-crystal diamond
material; a shank having a shank body composed of siliconized
silicon carbide; and a thermal interface composed of a metal
material to form a vacuum-brazed connection between the cutting
tool head and the shank body, the thermal interface being
configured for thermal and physical contact between the cutting
tool head to the shank body, wherein the thermal interface is
configured to act as a thermal conductor to transfer heat from the
cutting tool head to the shank body, and the shank body is
configured to act as a heat sink to transfer or conduct heat from
the thermal interface in order that an operational temperature of
the cutting edge is less than 482.degree. C.
[0066] Example Two may include the cutting tool of Example One,
wherein the cutting tool head is mechanically connected to and in
thermal communication with the shank via a vacuum-brazed thermal
interface.
[0067] Example Three may include the cutting tool of Example One,
wherein the cutting tool head contacts the holder body at a single
plane of contact interface where atomic bonds are shared.
[0068] Example Four may include the cutting tool of Example One,
wherein the metal material has a thermal conductivity that is
greater than the thermal conductivity of a holder material but less
than the thermal conductivity of diamond.
[0069] Example Five may include the cutting tool of Example One,
wherein the metal material comprises a silver alloy.
[0070] Example Six may include a cutting tool for ultra-precision
machining, comprising: a cutting tool head having a cutting edge to
contact a workpiece, the cutting tool head being composed of
diamond; a cutting tool head holder upon which the cutting tool
head is mounted, the cutting tool head holder being composed of a
holder material having a thermal conductivity of at least 225
W/m.sup.2K and a modulus of elasticity of not less than 340 GPa;
and a thermal interface configured for thermal and physical contact
between the cutting tool head to the cutting tool head holder, the
thermal interface being composed of a thermal conducting material
to form a vacuum-brazed connection between the cutting tool head
and the cutting tool head holder, and to also transfer heat from
the cutting tool head to the cutting tool head holder in order to
maintain an operational temperature of the cutting edge at less
than a predetermined temperature.
[0071] Example Seven may include the cutting tool of Example Six,
wherein the holder material comprises siliconized silicon
carbide.
[0072] Example Eight may include the cutting tool of Example Six,
wherein the predetermined temperature comprises 482.degree. C.
[0073] Example Nine may include the cutting tool of Example Six,
wherein the cutting tool head contacts the cutting tool head holder
at a single contact interface.
[0074] Example Ten may include the cutting tool of Example Six,
wherein the thermal conducting material comprises a metal
material.
[0075] Example Eleven may include the cutting tool of Example Ten,
wherein the metal material has a thermal conductivity that is
greater than the thermal conductivity of the cutting tool tip
holder and less than the thermal conductivity of diamond.
[0076] Example Twelve may include the cutting tool of Example
Eleven, wherein the metal material comprises silver or a silver
alloy.
[0077] The terms "coupled," "attached," or "connected" may be used
herein to refer to any type of relationship, direct or indirect,
between the components in question, and may apply to electrical,
mechanical, fluid, optical, electromagnetic, electromechanical or
other connections. In addition, the terms "first," "second," etc.
are used herein only to facilitate discussion, and carry no
particular temporal or chronological significance unless otherwise
indicated.
[0078] Those skilled in the art will appreciate from the foregoing
description that the broad techniques of the embodiments can be
implemented in a variety of forms. Therefore, while the embodiments
have been described in connection with particular examples thereof,
the true scope of the embodiments should not be so limited since
other modifications will become apparent to the skilled
practitioner upon a study of the drawings, specification, and
following claims.
* * * * *